1. Field of the Invention
The present invention relates to a printed wiring board group in which a plurality of printed wiring boards are arranged. More particularly, the present invention relates to a printed wiring board group composed of a plurality of printed wiring boards in which the electric resistance is uniform regardless of the configuration of the wiring pattern.
2. Description of the Related Art
Printed wiring boards used for electronic packaging are generally manufactured as follows. First, a photosensitive resin layer is formed on a surface of a conductive metal layer formed on an insulating film such as a polyimide film. Next, this photosensitive resin layer is exposed and developed in a desired form to form patterns. The printed wiring boards are manufactured by selectively etching the conductive metal layer by use of the patterns as a masking material, and thereby forming wires similar in shape to the masking material on the surface of the insulating film.
When such printed wiring boards are manufactured, a conductive metal layer is formed on an insulating film in a tape shape of several tens to several hundreds of meters, and a large number of printed wiring boards are sequentially manufactured. A large number of such printed wiring boards are considered as having identical characteristics.
Further, a large number of wires formed in one printed wiring board are regarded as having identical characteristics. A variation in electric resistance among wires is extremely small. For example, electric current flowing in a printed wiring connected to a liquid crystal panel is in the order of μA, and no attention has been paid to that there is a variation in electric resistance among the wires of the wiring.
However, in practice, for example, in a case in which the wires on the insulating film are different in length from one another, the electric resistances of the wires are different. When the thickness of a wiring pattern is uniform, an electric resistance of a wire of a printed wiring board is greater in proportion to a length of the wire, and is smaller in inverse proportion to a width of the wire.
Therefore, for example, in a printed wiring board in which an electronic part such as an IC chip is mounted near the center of the wiring board, the wires are shorter and have a smaller electric resistance as they are close to the center line, while the outer wires are long and have a larger electric resistance.
With focus on these respects, in Japanese Patent No. 2769130 (Patent Document 1), Japanese Patent No. 2525996 (Patent Document 2) and the like, there are disclosed inventions of a flexible printed circuit board in which the electric resistances of conductive paths are made substantially uniform by changing the width of wires. Namely, in Patent Documents 1 and 2, there are disclosed flexible printed circuit boards in which the width of the wires is adjusted such that the electric resistance of wires formed on the inner side and the electric resistance of wires formed on the outer side are made substantially uniform by making the width of the wires positioned on the outer side of the flexible printed circuit board larger than that of the wires positioned on the inner side.
In this way, when the wires in one flexible printed circuit board are changed in width depending on the length of the wires, it is possible to make the electric resistances of the wires substantially the same. Note that, in Patent Document 1 and Patent Document 2, that the electric resistances are substantially the same is defined as “the electric resistances of conductive paths are different from the intermediate value of the maximum resistance and the minimum resistance at most by 10 to 20% of the intermediate value” (refer to, for example, the paragraph [0017] in Patent Document 1).
In this way, when the electric resistances of the wires in one flexible printed circuit board are substantially uniform, input electric signals are uniformly varied by the electric resistances of the conductive paths, and the intensity of an output electric signal and the intensity of an input electric signal are less susceptible to the electric resistances of the conductive paths, which makes it possible to stably drive the flexible printed circuit board.
Meanwhile, printed wiring boards for electronic packaging (flexible printed circuit boards) have been used in various fields of computers, display apparatuses, various control devices and the like. Further, a great variety of electronic apparatuses have been controlled by semiconductor devices in which electronic parts have been mounted. Conventionally, such an electronic apparatus has been controlled by one electronic part. However, in accordance with the increasingly complicated and diversified control systems, there is a tendency to control one apparatus by using a plurality of electronic parts. In particular, in a display apparatus, a display panel is controlled by a large number of printed wiring boards disposed in parallel. An attempt has been made to increase an operation speed by a plurality of identical LSIs disposed in parallel. If a value of current flowing in such a circuit subtly changes, a malfunction may be caused.
Further, in the field of display apparatuses and the like, display elements in which pixels are made to emit light by emission of electrons have been developed. In such a display apparatus, it is necessary to increase a density of flowing electric current and a voltage applied because the efficiency of emission of electrons is low.
In such high-precision next-generation electronics, unlike conventional printed wiring boards on which electronic parts are merely mounted, a situation may occur in which the properties of the electronics are changed by the electric resistance of wires formed in printed wiring boards. For example, in a display apparatus, when there is a difference in supply current due to different electric resistances of wires, the brightness of pixels with low electric current tends to be low. Accordingly, a variation in the electric resistances of wires formed in a printed wiring board is desirably as small as possible.
Moreover, as described above, a printed wiring board is manufactured by forming a large number of wirings composed of a conductive metal on the surface of an insulating film of several tens to several hundreds of meters. Therefore, it is difficult to manufacture printed wiring boards having exactly the same properties even by the same method due to various factors such as etchant used for forming the wiring boards, and the metal forming the conductive metal layer (for example, the state of a copper layer). Moreover, even in the same lot, the properties of a printed wiring board manufactured at the beginning and a printed wiring board manufactured at the end are not exactly the same.
Accordingly, when the properties of adjacent printed wiring boards are different from one another, the problem will be brought about that, for example, shading and the like are easily generated on a display screen of a dominating printed wiring board. In particular, when a difference in the electric resistances between the outermost wire of a printed wiring board and the outermost wire of an adjacent printed wiring board is large, a brightness difference such as a clear separation line appears on the display apparatus. Further, in a case in which a plurality of identical electronic parts are disposed in parallel, the entire electronic apparatus can malfunction due to a slight difference in output current among the electronic parts.
In conventional printed wiring boards, the problem has been overcome to some extent by merely making the electric resistances of wires in one printed wiring board uniform. However, under the present situation where electronics are highly evolving, it is necessary not only to control the electric resistances of the wires in each printed wiring board, but also to maintain the relationship of the electric resistances of adjacent printed wiring boards in an appropriate range.
[Patent Document 1] Japanese Patent No. 2769130
[Patent Document 2] Japanese Patent No. 2525996